Electric furnace and electric melting and crystallizing method for minerals



June 21, 1955 R A. HUMPHREY 2,711,435

ELECTRIC FURNACE AND ELECTRIC MELTING AND CRYSTALLIZING METHOD FOR MINERALS Filed Feb. 20, 1953 2 Sheets-Sheet 2 I I I I I\ /MELT CARRIE CURRENT .75

\RESISTOR BURNS OUT 0 I I I I o no 3o 6O TI ME IN Houres 4, POWER bvpur 70 FUR/v4 c5 DURING 7YP/CAL Ram 50 MELT CARRIES CumeENT K I 30 REI5T0R Bums OUT 0 I I I I I 0 IO '20 3O 4O 5O 6O 70 TIME IN I'Ioures INVENTOR R4 014 R0 Affa/w /mf Y BY IITQNEY 2,711,435 lc Patented Ju 1955 United States Patent of this invention containing a batch of charge material prior to the start of the melting operation. 2711435 Fig. 2 shows a similar cross-section through the fur- ELECTRI FU AND ELECTRIC MELTING nace and through the batch of the material at the end AND cRYsrALLIZING METHOD FOR MIN- the melting Operation- ERALS Fig, 3 is a graph showing the furnace resistance during a typical run. Rmham Humphrey Upper Mantclalr Fig. 4 is a graph showing the power input to the fur- Application February 20, 1953, Serial No. 338,179 r1806 during it yp NHL The method of the invention com rises startin a con- 0 Chums (CL 13*23) ducting melt surrounded by sintered material wi t hin the (Gra t d under Tim 35, U 5, Code (1952 sec, 2 interior of a charge of material by directly contacting the material with a destructible or removable resistance heater positioned in the interior thereof between the the invention described herein may be manufactured 15 tips Of Vertically dispos'id eleltmdes, l'timoving or and used by or for the Government of the United States stroyi g the ffisisiof, melting a further Portion of i113 for governmental purposes without the payment to me charge and enlarging the area surrounded by said of any royalty thereon in accordance with the provisions sintered shell by resistance heating between said vertical ot the act of April 30, 1928 h, 460, 45 St t L, 467 electrodes using the melt as a conducting pool, whereby This invention relates to a method and apparatus for said shell of sintercd material maintained around the electric melting, and, more particularly, i concerned molten bath prevents volatilization from the bath, diswith a method and apparatus for the production of syncontinuing the melting while a substantial thickness of thetic minerals by melting mineral batche the unfused charge material still surrounds the melt, and

The production of certain minerals by crystallization COOiing afld crystallizing the: molten massfrom melts presents many problems due to the existing Referring now to the drawings for a more complete process and apparatus limitations. Molten mineral description of the invention and in partichlar to Figs. l are often highly corrosive to furnace Walls, and the comand 2 thereo the appara us comprises a suitable co r position of the melt may be difi'icult to control because min r whi h may e of heet metal as sho n, or of at volatilization of the charge constituents at the high refractory, or other material of Construction Since h melting temperature involved. Such problem a e pre- 3O container is thermally insulated from the interior of the sented, for example, in the synthesizing of fluorfurnace by unfused charge material, it is merely necesphlogopite rnicas by crystallization of tiuoro-silicate sary that the container have suificient strength to supbatches of suitable composition. Be au e f th high port the weight of the charge. A pair of substantially melting temperature of such batches, about 1400 (3,, horizontally disposed primary electrodes 12 extend into and their high fluorine content, ranging f m 9 t 13 the interior of the furnace in the lower portion thereof Percent h malts are very fl id d exgegdingdy corand are electrically insulated from the furnace walls by rosive. Therefore, one of the problems i th fi di f suitable electrical insulating material 14 of asbestos or a container for such melts that is both corrosion resistthe like- Secondary electrodes 16 @Xicnd Substantially a t a refractory. Although fireclay, platinum, vertically upwardly from the primary electrodes 12, prefgraphite, and silica carbide containers have been used, 40 erably to about the mid-point of the furnace body where containers constructed of these materials frequently tail one set of electrodes are employedv A resistor 13 of cardue to corrosion, are costly, can be constructed only of graphite, other material, designed IO be destroyed limited size, and may contaminate the product. Even y ihti heat produced 0? Otherwisfi be removfid after the in melting processes wherein a charge of material is chafgfi material begins to melt, is Positionfid YO $011MCt i i d as a b d Within the container excessive the tips of the two vertical electrodes. The primary and volatllization and loss of essential fluoride constituents n a y electrodes m y e made of gr phi car n. from the surface of the melt may take place to such an r h r a l du g m alsexteut that a mica product of the desired composition in Operating the fuinace, pcwderfid mu i0 02mm; b f f ne L melted is introduced into the furnace to form a batch as Accordingly, an object of this invention is to providg 5 shown at 20. The batch comprises substantially more an improved method and apparatus for the melting of of the material than is to be melted and y 90m i l material, for example, approximately ten times the amount of ma- Another object of the invention is to provide a method tfirifll to be melted- Y pp Power {0 the acct-T9463 and apparatus whereby a large homogeneous melt may and melting the batch adjacent to resist)? 13, an be produced without excessive volatilization of charge l Conducting P of melt Surmuflded y siflltllhil material and without the use of a sealed vessel. material is Produced along the P of the Another Object of the invmtion is to provide a mew tween the two vertically disposed secondary electrodes is. mg mgthod wherein tha batch of material acts as a Com As the batch melts, the level of the melt falls co moustainer for melting, serves as a highly efiective vapor seal, 1y as the melt ,occllples less Since UUCOHSUWQNJX and as an efficient thermal insulaton charge. Thus, 1n time, the res stor 18 18 exp zed to an Another object of the invention is to provide a method Oxldlzmg atmosphere Whlch graphite Or hm been used, causes it to be burned and consumed ever, by the time this happens, the pool of molt n ruse terial is large enough to contact the vertical ele odes 5 and the path of the electric current is directly through the molten material. As the melting proceeds, a thick shell 22 of sintered material forms within the charge as shown in Fig. 2. The internal electric resistance of the melt and apparatus for melting and crystallizing refractory materials that could not be melted in an open container.

Another object of the invention is to provide an electric furnace for melting materials of the type described whereby arcing between the electrodes of the furnace is avoided,

These and other Objects of the invention Will bacon: provides suflicient heat to continuously melt more of l, pp ffom the fOHOWiIlg description taken in 6011- surrounding sintered material and increase the size of the nection with the accompanying drawings where: molten bath and charge cavity. Since the melt is directly Fig. 1 shows a vertical section of the electric furnace in contact with the sintered batch, its temperature remains very close to the melting point 9f the surrounding material. On increasing the power input, the melting rate increases without a corresponding temperature rise. This prevents excessive heating of the melt which is undesirable in most instances and in some melts would cause marked decomposition of the product.

The size of the melt is controlled by the power input and the heating time. Upon obtaining a melt of the desired size, as may be determined from previous experb once, as indicated by thermocouples placed at strategic locations throughout the batch, or by observing the temperature of the furnace or of the container wall, the furnace can be turned off at the proper time, or the melt may be crystallized slowly by gradually reducing the power input to the furnace.

Fig. 2 shows a typical cross-section through the furnace at the end of the melting operation. It is apparent from this figure how the batch acts as a container, seal, and thermal insulator for the melt. When the furnace batch comprises constituents for producing mineral melts, as, for example, of synthetic mica, the sintered shell 22 is of a hard marble-like structure which forms at temperatures substantially lower than the melting tem perature. The vertical electrodes 16 burn off at their tips as the melt settles; however, since these electrodes are vertically positioned, they remain in contact with the melt throughout the entire operation. Thus, there is no need to move the electrodes or break the automatic vapor seal formed by the sintered material.

A substantial thickness of insulating charge material is retained between the furnace wall and the sintered shell 22. After the melt has solidified and has cooled sulfic-iently, the product is obtained by a simple mining operation. The remaining material may be crushed, sized, and used for making up additional charges.

It is understood that the powdered batch materials used in this process may consist of any single chemical, or mineral, or combination of materials or minerals required to produce a fusion product. The invention is especially suitable for the manufacture of synthetic fluorine micas such as those represented by the formula KzMgeAlaSisOzoFa KzMgeBzSieOzoFi which may be made by using such batch formulas as:

However, the method may also be applied to the synthesis of mineral products such as fiuor-tremolite, calcium fluoride (CaFz), and potassium magnesium fluoride. Ah though the invention is especially important when applied to batches containing a volatile phase such as fluorides, it may also be used for the synthesis of nonvolatile compositions such as, for example, cordierite (MgzAlrSiaOra), beta-spodumene (Ll3Al3Sl601a), wollastonite (CaSiOz) or monticellite (CaMgSiO4).

Figures 3 and 4 are illustrative of operating conditions within the furnace of this invention during a typical run. These data were compiled from an operation using a batch composition for production of fluor-phogopite mica using charge constituents as shown in Formula 1 above. Thus, it is noted that the resistor may burn out in three or four hours operation and the melt utilized to carry the current for the remainder of the operation which may comprise seventy hours or more.

It is evident that many modifications in the shape and size of the furnace are possible as well as in the arrangement of the electrodes which may be used.

The above-described invention obviates the necessity of providing special refractory containers for corrosive melts; it produces its own seal to retain volatile constituents (fluoride, chlorides, etc.) it provides an efficient and economical method for melting mineral batches. mainly because the charge material acts as a thermal insulator for the melt. By means of this invention, employing internal direct resistance melting, it is commercially feasible to melt large batches (tons) of fluorosilicate or other mineral batches containing volatile constituents, efiiciently and cheaply. Thus, production of synthetic fluor-phlogopite micas becomes commercially feasible. Of prime importance is the fact that the batch not only acts as a container for the melt but also serves as a highly effective vapor seal and as an etficient thermal insulator. The latter is especially important if the process is used for the growth of large crystals by slow cooling of the melt. Because of the effectiveness, as a vapor seal, of the sintered material that forms around the melt, it is possible to control the composition of large melts, as well as to avoid the escape of noxious gases to the atmosphere.

It will be appreciated from a reading of the foregoing specification that the invention herein described is susceptible of various changes and modifications without departing from the spirit and scope thereof.

What is claimed is:

1. In an electric furnace, a pair of electrodes each extending substantially vertically upward from the lower interior portion of the furnace and adapted to be covered by the charge material, and a horizontally disposed removable resistor connecting the tips of said electrodes.

2. in an electric furnace, horizontally disposed spaced primary electrodes projecting through the furnace walls, secondary electrodes connected to said primary electrodes each extending substantially vertically upward into the interior of the furnace, and a horizontally disposed rcmovable resistor connecting the tips of two said vertical electrodes.

3. In an electric furnace, horizontally disposed, spaced primary electrodes projecting through the furnace walls in the lower portion thereof, secondary electrodes connected to said primary electrodes extending substantially vertically upward to approximately the midsection of said furnace, a destructible resistance element connecting the tips of said vertical electrodes, the electrodes and resistance element adapted to be covered by a substantial depth of charge material, whereby a conducting melt may be formed by resistance heating through said resistance element, and resistance heating may be continued through said melt between said vertical electrodes after said resistance element has burned out.

4. A method of melting materials comprising forming a relatively large batch of said material, and heating said material from within the interior of said batch so as to form a melt surrounded by sintered material and unfused batch material, whereby the batch acts as a container for the melt, serves as a vapor seal, and thermally insulates the melt on all sides and continuing to melt said batch material by electrical resistance heating through the molten mass.

5. A method of melting volatile materials comprising starting a conducting melt in the interior of a batch of said material whereby a shell of sintered material forms around said melt and provides a vapor seal, melting a further portion of said material and enlarging the area encompassed by said sintered shell by electric resistance heating through said conducting melt, and discontinuing said melting while said sintered shell still surrounds the molten material.

6. A method for producing a melt by electric resistance heating comprising starting a conducting melt within the interior of a batch of charge material by direct resistance heating between the tips of spaced vertical electrodes positioned in said batch, continuing the heating and meling of charge material between said electrodes by resistance heating through said conducting melt, and discontinuing said heating while a substantial amount of unfused charge material remains between the melt and the walls of the batch container.

7. A method for producing mineral compositions comprising forming a batch of mineral constituents around spaced vertical electrodes connected at the tips thereof by a destructible resistance element, starting a conducting melt within the interior of said batch along said resistance element by electrical resistance heating of said element, continuing to melt said mineral constituents by electric resistance heating between said vertical electrodes through said conducting melt after said destructible resistance element has burned out, discontinuing the heating while said melt is still surrounded by sintered and unfused batch material, and cooling the melt so produced.

8. A method for the production of synthetic fluorine mica from materials providing mica constituents comprising forming a batch of said materials around spaced vertical electrodes connected at the tips thereof by a destructible resistance element, starting a conducting melt of said materials within the interior of said batch along said resistance element by electrical resistance heating of said element, continuing to melt siad materials by electric resistant heating between said vertical elecrodes through said conducting melt after said destructible resistance element has burned out, said heating within the interior of the batch forming a thick shell of sintered batch material around the melt which acts as a vapor seal, discontinuing the heating while said melt is still surrounded by sintered and unfused batch material, and cooling the melt to crystallize the mica product.

References Cited in the file of this patent UNITED STATES PATENTS 662,537 Koller Nov. 27, 1900 1,215,432 Trautmann Feb. 13, 1917 1,242,341 Fulton Oct. 9, 1917 1,621,446 Watson Mar. 15, 1927 

5. A METHOD OF MELTING VOLATILE MATERIALS COMPRISING STARTING A CONDUCTING MELT IN THE INTERIOR OF A BATCH OF SAID MATERIAL WHERBY A SHELL OF SINTERED MATERIAL FORMS AROUND SAID MELT AND PROVIDES A VAPOR SEAL, MELTING A FURTHER PORTION OF SAID MATERIAL AND ENLARGING THE AREA ENCOMPASSES BY SAID SINTERED SHELL BY ELECTRIC RESISTANCE HEATING THROUGH SAID CONDUCTING MELT, AND DISCONTINUING SAID MELTING WHILE SAID SINTERED SHELL STILL SURROUNDS THE MOLTEN MATERIAL. 